The profile of a slab, in particular, of a thin slab, must meet strict requirements in terms of its bowing or thickness taper when the slab leaves a continuous-casting installation and is passed to a rolling mill. For example, the required tolerances for a profile camber in a thin slab that is to be sent to a compact-strip-production (CSP) finishing train are in the range of 0.5% to 1% relative to the slab thickness. This means that the profile camber, e.g. in a 50 mm thick slab, must only measure between 0.25 mm and 0.5 mm. In addition, this profile camber should be as constant as possible over the entire length of the slab.
The reason for the profile camber in metal strips is what is called ferrostatic pressure that is present inside the metal strips that have not yet completely solidified and that presses from inside against the strand shell, thereby causing an outward crowning of the strand shell. It is true that this ferrostatic pressure is essentially constant inside the liquid part of the strand; however, the pressure increases as the liquid part of the metal strip becomes longer. This bulging of the strand shell caused by the ferrostatic pressure results in a loading of the guide rollers guiding the metal strip in the strand-guiding apparatus and transmit this loading through their bearings to a section frame on which the guide rollers are mounted by means of bearings. The load being transmitted typically results in a deflection or spring-back of the section frame, in particular, in the area of center bearings in the case of divided guide rollers. This undesirable spring-back of the section frame typically results in an undesirable change in the roller gap geometry, and thus in particular in an undesirably large profile camber in the metal strip guided in the strand-guiding means. Due to the undesirable profile camber, the metal strip often as a result no longer meets the requirements of the downstream rolling mill.
These problems are known in the art and are discussed, e.g. in EP 1,043,095 [U.S. Pat. No. 6,568,460]. Here it is emphasized that the most critical factor in maintaining the above-mentioned norms is to precisely control the roller gap geometry in the area of residual solidification in light of the above-referenced problems. For this purpose, this European patent teaches an approach whereby a force-exerting means in the form of a hydraulic cylinder is provided in the center region of the section frame, i.e. in the area of the center bearings so as to compensate for the above-referenced undesirable spring-back of the section frame.
These hydraulic cylinders are, however, very costly both in terms of acquisition as well as maintenance, and additionally entail ongoing operating costs, e.g. due to the regular consumption of electrical power.